It is well known that the adequacy of an underground pipeline cathodic protection system is assessed by measuring off-potential and DC current of a steel probe placed near the pipeline at depth (i.e., in the same environmental conditions). A saturated copper/copper sulfate reference electrode (CSE) is also installed in the vicinity of the steel probe. The steel probe, simulating a coating defect, is electrically connected to a pipeline. Off-potential without IR-drop is measured with respect to a CSE by interrupting the steel and a pipeline. Recently, this method termed instant-off method has been widely used to assess cathodic protection conditions.
FIG. 10 shows a schematic of the method for measuring pipe-to-soil potential with a steel probe. In FIG. 10, no. 1 depicts a pipeline, no. 2 a steel probe, no. 3 a CSE, no. 4 an ammeter in line 5 between a pipeline and a steel probe, no. 6 an on/off switch, no. 7 a potentiometer in line 8 between a steel probe and a CSE, no.9 an electrode for cathodic protection, no. 10 an anode, no. 11 a sacrificial anode, no. 12 a recorder, respectively.
In the above mentioned system, the adequacy of cathodic protection is assessed by measuring on-potential (pipe-to-soil) potential, off-potential, and current density. On-potential and current density are simultaneously obtained before interruption of the steel probe and a pipeline. Off-potential is obtained after disconnection of the steel probe and a pipeline for 0.6 to 1.0 sec in every 10 to 20 sec by using an on/off switch 6. Time variations of on-potential, offpotential, and current density are recorded using a recorder 12. An example of the output data by the field study is shown in FIG. 11.
However, some issues regarding the above mentioned method arise as follows:
1 Original waveforms regarding on-and off-potentials, and probe currents are not obtained due to the use of a recorder with lowpass filter. PA1 2 Off-potentials, DC and AC probe current densities are indispensable to assess the adequacy of cathodic protection. However, in the conventional method, off-potential of the steel probe is not accurately taken after disconnection of the steel probe and a pipe, when significant problem with depolarization is observed. Additionally, effective frequency in AC probe current density is not gained. PA1 3 It is impossible to calculate data when the time averaged values are required, for example. Because digital on-and off-potentials, and DC and AC probe current densities are not acquired in the system.
To overcome the above mentioned issues 1 through 3, the inventor of this patent has proposed the invention relating to the Japanese Application No. 8-345313. The patent describes the evaluation method for cathodic protection of underground pipelines with a steel probe and a CSE near the pipeline at a depth, by numerically analyzing on-and off-potentials, and probe currents that are collected with use of a computer. The steel probe is electrically connected to a pipeline. Frequency analysis of collected data is performed, then levels of on-and off-potentials, and probe currents at effective frequencies are assessed.
Recently, buried pipelines tend to run parallel to electric transmission lines or railways. In such a case, a powerfill magnetic field is generated between a pipeline and ground, then a considerable voltage is induced particularly in a well coated pipeline. The above mentioned patent (No. 8345313) does not directly refer to data regarding AC corrosion.
On the other hand, it is the current situation that the buried environment of pipeline is aggravating the situation more and more in recent years and the situation is increasingly becoming the cause for AC corrosion.